Each of the cells of all life forms, except viruses, contain ribosomes and therefore ribosomal RNA. A ribosome contains three separate single strand RNA molecules, namely, a large molecule, a medium sized molecule, and a small molecule. The two larger rRNA molecules vary in size in different organisms.
Ribosomal RNA is a direct gene product and is coded for by the rRNA gene. This DNA sequence is used as a template to synthesize rRNA molecules. A separate gene exists for each of the ribosomal RNA subunits. Multiple rRNA genes exist in most organisms, many higher organisms containing both nuclear and mitochondrial rRNA genes. Plants and certain other forms contain nuclear, mitochondrial and chloroplast rRNA genes. For simplicity of discussion hereinafter, the three separate rRNA genes will be referred to as the rRNA gene.
Numerous ribosomes are present in all cells of all life forms. About 85-90 percent of the total RNA in typical cell is rRNA. A bacteria such as E. coli contains about 10.sup.4 ribosomes per cell while a mammalian liver cell contains about 5.times.10.sup.6 ribosomes. Since each ribosome contains one of each rRNA subunit, the bacterial cell and mammalian cell contains 10.sup.4 and 5.times.10.sup.6, respectively, of each rRNA subunit.
Nucleic acid hybridization, a procedure well-known in the art, has been used in the prior art to specifically detect extremely small or large quantities of a particular nucleic acid sequence, even in the presence of a very large excess of non-related sequences. Prior art uses of nucleic acid hybridization are found, for example, in publications involving molecular genetics of cells and viruses, genetic expression of cells and viruses; genetic analysis of life forms; evolution and taxonomy or organisms and nucleic acid sequences; molecular mechanisms of disease processes; diagnostic methods for specific purposes, including the detection of viruses and bacteria in cells and organisms.
Probably the best characterized and most studied gene and gene product are the rRNA gene and rRNA, and the prior art includes use of hybridization of rRNA and ribosomal genes in genetic analysis and evolution and taxonomic classification of organisms and ribosomal gene sequences. Genetic analysis includes, for example, the determination of the numbers of ribosomal RNA genes in various organisms; the determination of the similarity between the multiple ribosomal RNA genes which are present in cells, determination of the rate and extent of synthesis of rRNA in cells and the factors which control them. Evolution and taxonomic studies involve comparing the rRNA gene base sequence from related and widely different organisms.
It is known that the ribosomal RNA gene base sequence is at least partially similar in widely different organisms, and that the DNA of E. coli bacterial ribosomal RNA genes hybridizes well with rRNA from plants, mammals, and a wide variety of other bacterial species. The fraction of the E. coli gene which hybridizes to these other species varies with the degree of relatedness of the organisms. Virtually all of the rRNA gene sequence hybridizes to rRNA from closely related bacterial species, while less hybridizes to rRNA from distantly related bacterial species, and even less with mammalian rRNA.
As with rRNAs, t-RNAs are present in all living cells, as well as in some viruses. t-RNA genes are present in chromosomal and plasmid DNAs of prokaryotes and in the DNA of eukaryotic cells, including the DNA of the nucleus, mitochondria and chloroplasts. Different t-RNA genes for one t-RNA species often exist in a single cell. t-RNA genes of mitochondria, nucleic and chloroplasts are quite different. Many virus genomes include genes for t-RNAs which are specific to the virus.
t-RNA molecules are direct gene products and are synthesized in the cells using the t-RNA gene as a template. The t-RNA is often synthesized as apart of a larger RNA molecule, and the t-RNA portion is then removed from this precursor molecule. After synthesis a fraction of the bases of the t-RNA molecule are chemically modified by the cell. A typical t-RNA molecule contains from 75-85 bases.
Numerous t-RNA molecules are present in all cells of all life forms, and usually about 10 percent of a cell's total RNA is composed of t-RNA, a typical bacterial cell containing about 1.5.times.10.sup.5 t-RNA molecules of all types. If each different king of t-RNA is equally represented in a bacterial cell then 2500 of each different t-RNA molecule is present in each cell. A typical mammalian liver cell contains about 10.sup.8 t-RNA molecules or an average of about 10.sup.6 copies per cell of each different t-RNA type.
During protein synthesis individual amino acids are aligned in the proper order by various specific t-RNAs, each amino acid being ordered by a different t-RNA species. Some amino acids are ordered by more than one t-RNA type.
There are certain viruses which contain t-RNA genes in their genomes, these genes produce virus specific t-RNA when the virus genome is active in a cell. These t-RNAs can also be present in multiple copies in each infected cell.
As with rRNA genes and rRNA, the prior art discloses use of hybridization of t-RNA and t-RNA genes in genetic analysis and evolution and taxonomic classification of organisms and t-RNA gene sequences. Genetic analysis includes, for example, the determination of the numbers of t-RNA genes in various organisms; the determination of the similarity between the multiple t-RNA genes which are present in cells; determination of the rate and extent of synthesis of t-RNA in ells and the factors which control them. Evolution and taxonomic studies involve comparing the t-RNA gene base sequence from related and widely different organisms.
And as with rRNA gene base sequences, it is known that an individual t-RNA gene base sequence is at least partially similar in different organisms. Total t-RNA shows this same type of relationship and bulk t-RNA from one species will hybridize significantly with t-RNA genes of a distantly related organism. Rat mitochondrial leucyl-t-RNA hybridized significantly with mitochondria DNA of chicken and yeast (Biochemistry (1975) 14, #10, p. 2037). t-RNA genes have also been shown to be highly conserved among the members of the bacterial family Enterobacteriaceae. Bulk t-RNA genes from E. coli hybridize well with t-RNA isolated from species representing different genes (J. Bacteriology (1977) 129, #3, p. 1435-1439). The fraction of the E. coli t-RNA/gene which hybridizes to these other species varies with the degree of relatedness of the organisms. A large fraction of the E. coli t-RNA gene sequence hybridizes to t-RNA from a closely related species while much less hybridized to rRNA from distantly related species.
The extent of conservation of the t-RNA gene sequences during evolution is not as great as that for the rRNA gene sequences. Nonetheless the t-RNA gene sequences are much more highly conserved than the vast bulk of the DNA sequences present in cells.
The sensitivity and ease of detection of members of specific groups of organisms by utilizing probes specific for the rRNA or t-RNA of that group of organisms is greatly enhanced by the large number of both rRNA and t-RNA molecules which are present in each cell. In addition the hybridization test is made significantly easier since RNA molecules present in cells are single stranded. Thus a denaturation step, such as must be used for a hybridization test which detects any fraction of cell DNA, is not necessary when the target molecule is RNA. Probes specific for other classes of cell nucleic acids, besides rRNA or t-RNA, may be used to specifically detect, identify and quantitate specific groups of organisms or cells by nucleic acid hybridization. Thus, other classes of RNA in prokaryotic cells include messenger RNA (hereinafter mRNA), and RNA sequences which are part of a variety of precursor molecules. For example rRNA is synthesized in the bacteria E. coli as a precursor molecule about 6000 bases long. This precursor molecule is then processed to yield the rRNA subunits (totaling about 4500 bases) which are incorporated into ribosomes and the extra RNA sequences (1500 bases in total) which are discarded. t-RNA molecules and ribosomal 5S RNA are also synthesized and processed in such a manner.
In prokaryotic cells infected by viruses there is also virus specific mRNA present. The mRNAs of certain prokaryotic viruses are also synthesized as a precursor molecule which contains excess RNA sequences which are trimmed away and discarded.
Many of the prokaryotic mRNAs and virus mRNAs are present up to several hundred times per cell while thousands of the excess RNA sequences present in rRNA or t-RNA precursor molecules can be present in each cell.
Eukaryotic cells also contain precursor mRNA, as well as precursor rRNA and t-RNA, molecules which are larger than the final rRNA or t-RNA molecules. In contrast to prokaryotes, many newly synthesized eukaryotic mRNA molecules are much larger than the final mRNA molecule and contain excess RNA sequences which are trimmed away and discarded. Another class of RNA present in eukaryotic cells is heterogeneous nuclear RNA (hereinafter known as hn-RNA), which is a diverse class of RNA which contains mRNA precursor molecules (which leave the nucleus for the cytoplasm where protein synthesis occurs) and a large amount of RNA which never leaves the nucleus. This fraction also contains a small fraction of double strand RNA. Eukaryotic nuclei also contain small RNA molecules called small nuclear RNA (hereinafter snRNA), varying in length from 100-200 bases.
The abundance, or number of copies per cell, of different mRNA molecules varies greatly. This varies from a complex class of mRNA molecules which are present only 1-2 times per cell, to the moderately abundant class of RNA molecules which are present several hundred times per cell, to the superabundant class of RNA molecules which may be present 10.sup.4 or more times per cell. Many of the RNA sequences present in hnRNA are also very abundant in each cell. The RNA sequences present in the precursor RNA molecules for rRNA, t-RNAs and many mRNAs are also very abundant in each cell. Individual snRNA sequences are extremely abundant and may be present from 10.sup.4 to 10.sup.6 times per cell.
Eukaryotic cells are also infected by viruses which produce virus specific mRNA and in many cases virus specific precursor mRNA molecules which contain RNA sequences not present in the mature mRNA molecule. The individual virus specific mRNA and precursor RNA molecules vary in abundance from complex (1-2 copies per cell) to superabundant (around 10.sup.4 copies per cell).
My invention also relates therefore, to a method for specifically and sensitively detecting, identifying and quantitating organisms, as well as, viruses, present in cells. More particularly, the method is useful for sensitively detecting, identifying and quantitating any member of different sized categories of organisms, eukaryotic cells, viruses, and in some cases previously unknown organisms containing mRNA, hnRNA, snRNA or excess RNA molecules present in rRNA, t-RNA, mRNA, or hnRNA molecules.
This invention therefore has broad application to any area in which it is important to determine; the presence or absence of living organisms, or viruses present in cells; the state of genetic expression of, an organism, cell, virus present in a cell, or groups of cells or prokaryotic or eukaroytic organisms. Such areas include medical, veterinary, and agricultural diagnostics and industrial and pharmaceutical quality control.
The invention involves a method for using specifically produced nucleic acids complementary to, not only rRNA and t-RNA, but also to specific sequences or populations of different sequences of the RNA class mRNA or hnRNA or snRNA or the class of RNA sequences (hereinafter known as precursor specific RNA sequences or psRNA) which are present only in the precursors mRNA, t-RNA, hnRNA or snRNA molecules and not in mature mRNA, rRNA, t-RNA, hnRNA or snRNA molecules, to detect, identify and quantitate specific organisms, groups or organisms, groups of eukaryotic cells or viruses in cells, by the process of nucleic acid hybridization.
My invention and the novelty, utility and non obviousness thereof can be more clearly understood and appreciated when considered in the light of the additional representative background information hereinafter set out, comprising this art:
1. mRNAs, and psRNAs are present in all organisms and cells, hnRNAs and snRNAs are present only in eukaryotic cells. Cell organelles which contain DNA, including mitochondria and chloroplasts, also contain mRNA, psRNA, rRNA, and t-RNA.
2. A typical bacterial cell contains more than a thousand genes, the vast majority of which code for a specific protein. A mammalian cell contains over 10,000 genes each of which can produce RNA. Any gene has the potential to produce multiple copies of RNA in a cell. Each specific RNA molecule produced is a direct gene product.
3. Many different mRNA sequences can be present in each organism or cell. The individual cells of a multicelled organism may have different mRNA sequences present in each cell or in different groups of cells.
Many different hnRNA, psRNA, and snRNA sequences can be present in each cell or group of cells of a eukaryotic organism.
Cells infected with a specific virus can have present within them a variety of different types of virus specific mRNA and ps RNA.
4. The number of copies (hereinafter the abundance) of a specific mRNA in a prokaryotic cell varies from zero to several hundred. The abundance of a specific psRNA sequence in a prokaryotic organism or cell can be 10 to 20 times higher.
The abundance of a specific mRNA molecule in a eukaryotic cell ranges from 1-2 to greater than 10.sup.4 per cell.
The abundance of a specific hnRNA sequence in a eukaryotic cell ranges from 1-2 to greater than 10.sup.4 per cell.
The abundance of a specific snRNA molecule in a eukaryotic cell varies from 10.sup.4 to 10.sup.6 per cell.
The abundance of a specific psRNA sequence in a eukaryotic cell varies from 1-2 to over 10.sup.4 per cell.
5. In many eukaryotes, RNA of various types is produced from the repeated sequence fractions of the DNA. This can result in a population of abundant RNA molecules whose sequences are similar but not identical to one another. A probe complementary to one of these RNA molecules will, however, hybridize with all of the other similar RNA molecules.
6. The gene sequences which code for the various individual mRNAs. psRNAs, hnRNAs and snRNAs of viruses and living organisms, have been conserved to varying degrees through evolution. The vast majority of these sequences are much less conserved than t-RNA sequences. Some of the sequences, however, are highly conserved. For example the gene which codes for histone mRNA is very highly conserved through evolution and the histone gene sequence is quite similar in widely different organisms.
The lack of conservation in the DNA sequences of many of these RNAs allows the production of probes which can readily distinguish between closely related organisms or viruses.
A large number of studies have been done on various mRNAs, hnRNAs, snRNAs and psRNAs (see Gene Expression, vol. 1 and 2, by B. Lewin, in references). These include hybridization of these RNAs in studies on genetic analysis, regulation and evolution, in prokaryotic and eukaryotic organisms and viruses.